Therapeutic approaches to various disorders, diseases and conditions may contemplate the delivery of a biologically active protein into a cell, so that the protein can perform its biological function within a cellular context.
Existing methods for delivery of biologically active proteins into cells include physical methods such as microinjection [6, 7] and electroporation [8, 9], which may prove difficult to apply in vivo.
Molecular techniques include conjugation of a protein transduction domain (PTD) to an active protein for mediation of cellular uptake of the protein. The three most actively studied PTDs are derived from Drosophilia anntennapedia peptide, HSV-VP22 protein and the HIV-TAT protein transduction motif. Transduction across cell membranes by these PTDs occurs through a mechanism that presently is not well understood, but studies have shown that the peptide and protein delivery have a strong correlation with the content and distribution of positively charged lysine and arginine residues in the PTDs [10, 11].
Another approach in rational drug delivery research that is becoming increasingly popular involves cationic lipids and polymers. For example, polyethylenimine (PEI)-conjugated proteins are able to enter cells based on ionic charge interactions [12]. The conjugation of proteins with PEI must be conducted under mild conditions to protect proteins from denaturation. Moreover, cytotoxicity of PEI, especially PEI having high molecular weight, also limits its in vivo applications.
There exists a need for alternative approaches for delivering a protein, including a biologically active protein or an antibody, into a cell.